1. Field of the Invention
The present invention relates to an apparatus provided in measurement equipment for calibrating the DC voltage level of the measurement equipment and, more particularly, to a calibration apparatus that reduces the effect on the measurement accuracy when the measurement equipment measures a high-frequency signal.
2. Description of the Prior Art
A semiconductor tester provides a calibration apparatus at the input thereof for stabilizing and accurately measuring the signal to be measured. FIG. 1 is a schematic drawing of a conventional semiconductor tester. The calibration apparatus provided therein is explained.
In FIG. 1, the semiconductor tester 100 is comprised of an input terminal 110 for receiving the signal to be measured, measurement equipment 120 for measuring the signal to be measured, and a calibration apparatus 130 for calibrating the DC voltage level of the measurement equipment. The calibration apparatus 130 is comprised of a switch 131 and a reference voltage source 132. The switch 131 is provided between the output of the reference voltage source 132 and the input of the measurement equipment 120 and connects or disconnects the reference voltage source 132 and the measurement equipment 120. The reference voltage source 132 outputs a DC voltage for calibration.
In this specification, the voltage for calibration is also referred to as the calibration voltage.
When the semiconductor tester 100 measures a signal, the switch 131 enters the open state. Then the measurement equipment 120 measures the signal received at the input terminal 110. When the semiconductor tester 100 self calibrates, the switch 131 enters the conducting state. The measurement equipment 120 reads in a specified reference voltage output from the reference voltage source 132. Furthermore, the measured voltage and the output voltage of the reference voltage source 132 are compared. If a difference arises between the two voltages, the measurement equipment 120 is calibrated so that the difference disappears, and the calibration ends.
Problems related to the measurement accuracy are produced when a high-frequency signal is measured in a semiconductor tester configured as described above. In general, a switch has a signal line between the terminal for connecting to an external circuit and the actual switch mechanism, and the connecting side in the switch mechanism also becomes a signal line. This kind of signal line forms an open stub when the switch is in the open state. The open stub behaves as a capacitive load or an inductive load and has an effect on the frequency characteristics of a circuit or equipment connected by the open stub.
Usually, measurement equipment that handles high-frequency signals is adjusted so that the input impedance thereof is 50 ohms. Since the resistance of the signal line from the reference voltage source 132 to the measurement equipment 120 has a value that cannot be ignored compared to the input impedance (50 ohms), the voltage applied to the measurement equipment 120 cannot be controlled with high accuracy.
To solve the above-mentioned problems, for example, the switch 131 can be changed to a single-pole, double-throw (SPDT) switch. FIG. 2 shows a semiconductor tester including a calibration apparatus using an SPDT switch. The semiconductor tester 200 in FIG. 2 comprises an input terminal 210 for receiving the signal to be measured, measurement equipment 220 for measuring the signal to be measured, and a calibration apparatus 230 for calibrating the DC voltage level of the measurement equipment. The calibration apparatus 230 comprises a switch 231 that is a high-frequency SPDT switch and a reference voltage source 232. When the semiconductor tester 200 measures a signal, the switch 231 enables conduction between the input terminal 210 and the measurement equipment 220. When the semiconductor tester 200 is self calibrating, the switch 231 enables conduction between the reference voltage source 232 and the measurement equipment 220. The SPDT switch, for example, can be an RF switch 8762A from Agilent Technologies.
If a high-frequency SPDT switch as described above is used, the open stub problem is eliminated. However, this kind of switch is large and expensive, and has a short lifetime. It is desirable that the semiconductor tester be able to operate continuously for a long time period. Furthermore, since a smaller sized and less expensive switch has been demanded in recent years, new solution techniques are desired.